fix: prevent asset conflicts between React and Grid.js versions

Add coexistence checks to all enqueue methods to prevent loading
both React and Grid.js assets simultaneously.

Changes:
- ReactAdmin.php: Only enqueue React assets when ?react=1
- Init.php: Skip Grid.js when React active on admin pages
- Form.php, Coupon.php, Access.php: Restore classic assets when ?react=0
- Customer.php, Product.php, License.php: Add coexistence checks

Now the toggle between Classic and React versions works correctly.

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

node_modules/resolve/test/default_paths.js

@@ -0,0 +1,49 @@
+'use strict';
+
+var path = require('path');
+var test = require('tape');
+var mockProperty = require('mock-property');
+
+var homedirPath = require.resolve('../lib/homedir');
+var asyncPath = require.resolve('../async');
+var libAsyncPath = require.resolve('../lib/async');
+var syncPath = require.resolve('../sync');
+var libSyncPath = require.resolve('../lib/sync');
+
+function mockNullHomedir(t) {
+    t.teardown(mockProperty(require.cache, homedirPath, {
+        value: { id: homedirPath, filename: homedirPath, loaded: true, exports: function () { return null; } }
+    }));
+}
+
+test('async: null homedir does not throw', function (t) {
+    t.plan(2);
+
+    mockNullHomedir(t);
+    t.teardown(mockProperty(require.cache, asyncPath, { 'delete': true }));
+    t.teardown(mockProperty(require.cache, libAsyncPath, { 'delete': true }));
+
+    var resolve = require('../lib/async');
+
+    var dir = path.join(__dirname, 'resolver');
+
+    resolve('./baz', { basedir: dir }, function (err, res) {
+        t.error(err, 'no error');
+        t.equal(res, path.join(dir, 'baz', 'quux.js'), 'resolves correctly with null homedir');
+    });
+});
+
+test('sync: null homedir does not throw', function (t) {
+    mockNullHomedir(t);
+    t.teardown(mockProperty(require.cache, syncPath, { 'delete': true }));
+    t.teardown(mockProperty(require.cache, libSyncPath, { 'delete': true }));
+
+    var resolveSync = require('../lib/sync');
+
+    var dir = path.join(__dirname, 'resolver');
+
+    var res = resolveSync('./baz', { basedir: dir });
+    t.equal(res, path.join(dir, 'baz', 'quux.js'), 'resolves correctly with null homedir');
+
+    t.end();
+});